Powders containing carbon, silicon and boron have been produced up until now using three types of techniques:                the mechanical alloying technique;        the technique using a solution route;        the technique using a thermal route.        
The mechanical alloying technique consists of mechanically grinding submicron-size powders of silicon carbide (SiC) and boron carbide (B4C), in a device of crusher or ball mill type, for a sufficient time (often several hours) in an attempt to obtain a closely mixed powder having a silicon carbide phase and a boron carbide phase. However, no obtaining of powders of nanometric size has been reported using this technique. Moreover, the mechanical milling of powders may generate pollution of the resulting powders by elements originating from the milling device. As a result, the powders derived from this process cannot be used in applications requiring powders of high purity.
According to the solution technique, powders comprising carbon, silicon and boron are synthesized via sol-gel process using silicon-based precursors, carbon-based precursors and boron-based precursors. Such is the case in particular with the process described in Zhi-min et al. (Trans. Nonferrous Met. Soc. China, 16 (2006), 470-473), which respectively comprises:                a step to mix tetraethoxysilane, ethanol, sucrose and water;        a step to add tributyl borate to the resulting mixture, the pH being maintained until homogenization at a value of between 3 and 4;        a drying step in an oven at 60° C. to obtain a gel;        a carbothermal reducing step of the formed silica.        
According to the technique via thermal route, the powders comprising carbon, silicon and boron can be prepared from gaseous precursors using different heat sources such as laser (in which case the term laser pyrolysis is used) or a plasma.
For example, Vassen et al. (Journal of Materials Science 31 (1996) 3623-3637), using laser pyrolysis, synthesized powders comprising carbon, silicon and boron from a mixture of precursors: SiH4—C2H4—B2H6 with a boron content not exceeding 4% by weight. This synthesis mode, inter alia, has the disadvantage of using diborane B2H6, which is an unstable gas of high cost, and on this account difficult to use for producing powders having higher boron contents than the above-mentioned content.
Guo et al. (Journal of Materials Science, 32 (1997), 5257-5269) synthesized powders comprising carbon, silicon and boron through the use of a thermal plasma from a mixture comprising solid silicon, boron trichloride BCl3 and methane CH4. The powders obtained with this synthesis mode comprise less than 4% by weight of boron and finally do not give any boron carbide phase (B4C) but boron nitride. The formation of boron nitride is generated by reaction of the nitrogen N2 derived from the plasma on the transiently formed boron carbide as per the following reaction:(½)B4C+N2→2BN+(½)C
In addition, the powders obtained with this method are of submicron size.
Other authors have endeavored to synthesize powders comprising carbon, silicon and boron using thermal plasma, in particular using a radiofrequency plasma torch, such as Saiki et al. in U.S. Pat. No. 4,847,060 starting from a mixture of reagents such as a mixture of SiCl4, CH4 and BCl3, and arriving at submicron powders having poor boron distribution in the particles of boron carbide and a boron content not exceeding 5% by weight.
On the basis of the drawbacks of prior art methods, the authors hereof have set themselves the objective of proposing a method for preparing powders comprising carbon, silicon and boron in the form of silicon carbide and boron carbide and/or boron alone, the said method being of simple implementation and low cost and which, by means of a reasoned combination of steps and reagents, allows powders to be obtained able to meet the following characteristics:                nanometric size of the constituent particles of the powder;        narrow size distribution;        a boron content which may be high (for example possible reaching 30% by weight relative to the total weight of the other elements present in the powder);        good dispersion of the silicon carbide and boron carbide phases;        controlled composition of the powder, in particular regarding the ratios Si/B, Si/C and B/C.        